Conventionally, keyless entry systems have been proposed in which the unlocking and locking of a lock mechanism, which locks the doors of a vehicle, the luggage compartment door or the like with the doors in a closed state, is carried out without using a key. This keyless entry system is equipped with a portable transmitter. A switch for indicating unlocking or locking is provided at the transmitter. A code signal which is predetermined for each vehicle is transmitted in accordance with operation of the switch by the user.
The code signal transmitted from the transmitter is received by an antenna attached to the vehicle. By checking the received code signal with a code signal stored in advance, a determination is made as to whether the correct code signal has been received. In a case in which the correct code signal has been received, the lock mechanism is unlocked or locked by a driving means such as a solenoid or the like. In this way, locking and unlocking of the lock mechanism can be effected without using a manual key.
In a keyless entry system, the radio waves outputted from the transmitter are weak waves (500 .mu.V/m or less) which do not require a license stipulated by law. As a result, in order to reliably operate the keyless entry system in accordance with the designations of the user, efficient reception of the radio waves outputted from the transmitter is desired. In order to efficiently receive the radio waves in this way, it is preferable that the antenna is provided so as to project from the vehicle body. However, drawbacks arise in that, when the antenna is disposed in this way, the appearance of the vehicle deteriorates, and the workability when the antenna is attached to the vehicle is also low.
These drawbacks with respect to appearance and workability are solved by providing the antenna within the receiving unit. However, the position at which the receiving unit is disposed is generally a region which is difficult for the radio waves to reach and which has a low electrical field strength, such as within the luggage compartment space, under a seat, at the reverse side of the instrument panel or the like. Therefore, there are cases in which it is difficult to receive the radio waves.